The substructure of tight junctions is investigated by direct freezing techniques that avoid any chemical fixation and serve to increase the resolution of individual membrane components. The backbone of the tight junction is a pair of rod-shaped structures embedded in the central lipophilic domain of each of the paired component membranes. This model replaces the previous view that tight junctions are comprised of rows of intramembrane proteins. Instead, the rod-shaped structures are now interpreted as inverted cylindrical micelles of membrane lipids. A similar model now can be applied to tight junctions in invertebrates. Evidence for this model is being gathered from investigations of pure lipid bilayer systems which are induced to form nonplanar micellar phases by addition of calcium ion. Cylindrical micelles identical to these postulated at tight junctions are found embedded in these lipid bilayers. Another approach to determining tight junction structure is to explore the true inner surface of naturally occurring tight junctions by deep-etching. A filamentous structure has been found on this surface of the membrane which is coextensive with the cylindrical micelle. This structure is a good candidate for the protein associated with tight junctions, and may explain how cylindrical micelles are stabilized in certain regions of the cell membrane. These results lead to an understanding of how tight junctions serve in the blood-brain barrier system to prevent small charged solutes from entering the brain. Similar techniques are being applied to understand the substructure of specific glial membrane structures which are regarded as components of the blood-brain barrier system.